1. Field of the Invention
The present invention relates to a semiconductor optical modulator and an integrated optical circuit device used in trunk line optical communication systems. More particularly, it relates to a semiconductor optical modulator which eliminates phase modulation generated in conjunction with intensity modulation of light and an integrated optical circuit device with the semiconductor optical modulator.
2. Description of the Related Art
FIG. 6A is a perspective view of a semiconductor optical modulator of the prior art, and FIG. 6B is a cross sectional view taken along line, VIB--VIB of FIG. 6A. In the drawing, numeral 1 denotes an n-InP substrate, 2 denotes an n-InP buffer layer, 3 denotes an n-InP cladding layer, 4 denotes an n-InGaAsP separate confinement layer, 5 denotes an MQW light absorption layer comprising InGaAs well and InGaAsP barrier layers, 6 denotes a p-InGaAsP separate confinement layer, 7 denotes a first p-InP cladding layer, 8 denotes a second p-InP cladding layer, 9 denotes a p-InGaAs contact layer, 10 denotes a semi-insulating InP-embedded layer, 11 denotes a SiO.sub.2 insulation film, 12 denotes a Cr/Au electrode, 13 denotes an Au plating layer, 14 denotes an AuGe/Ni/Ti/Pt/Ti/Pt/Au electrode and 15 denotes a plated Au layer.
In the semiconductor optical modulator of the electro-absorption type as described above, applying a reverse bias to the light absorption layer 5 causes the absorption spectrum of the light absorption layer 5 to shift toward longer wavelengths due to the quantum confined Stark effect. This causes a change in the absorption spectrum of the light absorption layer 5 for light of a given wavelength, thus making it possible to apply intensity modulation of light transmitted through the semiconductor optical modulator.
It is known that, as the absorption spectrum of the light absorption layer 5 is changed by varying the voltage and hence the electric field applied to a semiconductor optical modulator, refractive index of the light absorption layer 5 also changes according to the Kramers-Kronig relation.
Such a change in the refractive index causes modulation of phase, as well as intensity, of output light of the semiconductor optical modulator as indicated by the following equation (1). EQU d.phi./dt=(2.pi./.lambda.).multidot.(dn/dt).multidot.L (1)
where .phi. is phase of light, t is time, .lambda. is wavelength of light in vacuum, n is refractive index of the semiconductor optical modulator and L is length of the modulator.
FIG. 7 shows measured values of an .alpha. parameter which is the ratio of change in refractive index to change of absorption by the light absorption layer 5 as a function of the applied voltage. As will be clear from FIG. 7, because the .alpha. parameter is not always zero in the range of the applied voltage, changing the applied voltage and thereby changing the absorption spectrum of the light absorption layer 5 causes the refractive index of the light absorption layer 5 to change accordingly.
As a result, light transmitted through the light absorption layer 5 is subject not only to intensity modulation but also accompanying phase modulation which takes place in the light absorption layer 5, thus giving rise to the so-called chirping phenomenon.
While output light which has been modulated by a semiconductor optical modulator is transmitted through a transmission path which involves wavelength dispersion, such as an optical fiber, there has been a problem that propagation speed in such a transmission path differs depending on the wavelength of light, thus resulting in deterioration of the waveform of the light signal during transmission.
To counter this problem, Japanese Patent Kokai Publication No. 3-293622 discloses a configuration of semiconductor optical modulator wherein an optical waveguide region is provided along the direction of light propagation on a side wall of the semiconductor optical modulator thereby to mitigate the variation of the refractive index. With this configuration, however, it is necessary to provide the optical waveguide region on the side wall of the semiconductor optical modulator to apply a voltage to the light absorption layer, thus making the manufacturing process complicated which, together with having the difficulty of forming the optical waveguide layer with a uniform thickness, makes it difficult to apply the configuration to a mass production process.